Single crystalline phase catalyst

ABSTRACT

An orthorhombic phase mixed metal oxide is produced selectively in quantitative yield.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This non-provisional application is a divisional of non-provisional U.S.patent application Ser. No. 10/117,859 filed Apr. 8, 2002 now U.S. Pat.No. 6,740,620 benefit of which is claimed under 35 U.S.C. §120 and whichin turn claims benefit under 35 U.S.C. §119(e) of U.S. provisionalApplication No. 60/286,235 filed Apr. 25, 2001, priority benefit ofwhich is also claimed for the present divisional application.

FIELD OF THE INVENTION

The present invention relates to a catalyst for the oxidation ofalkanes, or a mixture of alkanes and alkenes, to their correspondingunsaturated carboxylic acids by vapor phase catalytic oxidation and,more particularly, to a method of making the catalyst and to a processfor the vapor phase catalytic oxidation of alkanes, or a mixture ofalkanes and alkenes, to their corresponding unsaturated carboxylic acidsusing a catalyst prepared by the present method of making a catalyst.The present invention also relates to a process for the vapor phasecatalytic oxidation of alkanes, or a mixture of alkanes and alkenes, inthe presence of ammonia, to their corresponding unsaturated nitritesusing a catalyst prepared by the present method of making a catalyst.

BACKGROUND

Nitriles, such as acrylonitrile and methacrylonitrile, have beenindustrially produced as important intermediates for the preparation offibers, synthetic resins, synthetic rubbers, and the like. The mostpopular method for producing such nitrites is to subject an olefin suchas propene or isobutene to a catalytic reaction with ammonia and oxygenin the presence of a catalyst in a gaseous phase at a high temperature.Known catalysts for conducting this reaction include a Mo—Bi—P—Ocatalyst, a V—Sb—O catalyst, an Sb—U—V—Ni—O catalyst, a Sb—Sn—Ocatalyst, a V—Sb—W—P—O catalyst and a catalyst obtained by mechanicallymixing a V—Sb—W—O oxide and a Bi—Ce—Mo—W—O oxide. However, in view ofthe price difference between propane and propene or between isobutaneand isobutene, attention has been drawn to the development of a methodfor producing acrylonitrile or methacrylonitrile by an ammoxidationreaction wherein a lower alkane, such as propane or isobutane, is usedas a starting material, and it is catalytically reacted with ammonia andoxygen in a gaseous phase in the presence of a catalyst.

In particular, U.S. Pat. No. 5,281,745 discloses a method for producingan unsaturated nitrile comprising subjecting an alkane and ammonia inthe gaseous state to catalytic oxidation in the presence of a catalystwhich satisfies the conditions:

(1) the mixed metal oxide catalyst is represented by the empiricalformulaMo_(a)V_(b)Te_(c)X_(x)O_(n)wherein X is at least one element selected from the group consisting ofniobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium,manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium,platinum, antimony, bismuth, boron and cerium and, when a=1, b=0.01 to1.0, c=0.01 to 1.0, x=0.01 to 1.0 and n is a number such that the totalvalency of the metal elements is satisfied; and

(2) the catalyst has X-ray diffraction peaks at the following angles(±0.3°) of 2θ in its X-ray diffraction pattern: 22.1°, 28.2°, 36.2°,45.2°and 50.0°.

Similarly, Japanese Laid-Open Patent Application Publication No.6-228073 discloses a method of nitrile preparation comprising reactingan alkane in a gas phase contact reaction with ammonia in the presenceof a mixed metal oxide catalyst of the formulaW_(a)V_(b)Te_(c)X_(x)O_(n)wherein X represents one or more elements selected from niobium,tantalum, titanium, aluminum, zirconium, chromium, manganese, iron,ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony,bismuth, indium and cerium and, when a=1, b=0.01 to 1.0, c=0.01 to 1.0,x=0.01 to 1.0 and n is determined by the oxide form of the elements.

Unsaturated carboxylic acids such as acrylic acid and methacrylic acidare industrially important as starting materials for various syntheticresins, coating materials and plasticizers. Commercially, the currentprocess for acrylic acid manufacture involves a two-step catalyticoxidation reaction starting with a propene feed. In the first stage,propene is converted to acrolein over a modified bismuth molybdatecatalyst. In the second stage, acrolein product from the first stage isconverted to acrylic acid using a catalyst composed of mainly molybdenumand vanadium oxides. In most cases, the catalyst formulations areproprietary to the catalyst supplier, but, the technology is wellestablished. Moreover, there is an incentive to develop a single stepprocess to prepare the unsaturated acid from its corresponding alkene.Therefore, the prior art describes cases where complex metal oxidecatalysts are utilized for the preparation of unsaturated acid from acorresponding alkene in a single step.

Japanese Laid-Open Patent Application Publication No. 07-053448discloses the manufacture of acrylic acid by the gas-phase catalyticoxidation of propene in the presence of mixed metal oxides containingMo, V, Te, O and X wherein X is at least one of Nb, Ta, W, Ti, Al, Zr,Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, Li, Na, K, Rb, Cs andCe.

Commercial incentives also exist for producing acrylic acid using alower cost propane feed. Therefore, the prior art describes caseswherein a mixed metal oxide catalyst is used to convert propane toacrylic acid in one step.

U.S. Pat. No. 5,380,933 discloses a method for producing an unsaturatedcarboxylic acid comprising subjecting an alkane to a vapor phasecatalytic oxidation reaction in the presence of a catalyst containing amixed metal oxide comprising, as essential components, Mo, V, Te, O andX, wherein X is at least one element selected from the group consistingof niobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium,manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium,platinum, antimony, bismuth, boron, indium and cerium; and wherein theproportions of the respective essential components, based on the totalamount of the essential components, exclusive of oxygen, satisfy thefollowing relationships:

-   0.25<r(Mo)<0.98, 0.003<r(V)<0.5, 0.003<r(Te)<0.5 and 0.003<r(X)<0.5,    wherein r(Mo), r(V), r(Te) and r(X) are the molar fractions of Mo,    V, Te and X, respectively, based on the total amount of the    essential components exclusive of oxygen.

The mixed metal oxide catalysts useful in the preparation of unsaturatedcarboxylic acids and unsaturated nitriles, as delineated above, can formmore than one phase from the same starting materials under the sameconditions. Often one phase performs better than the others, so it isdesirable to prepare a catalyst that contains that phase exclusively,with the other phases substantially absent.

The aforementioned mixed metal oxide catalysts useful in the preparationof unsaturated carboxylic acids and unsaturated nitriles form at leastthree phases: a hexagonal phase (phase A), which is active butrelatively unselective; an orthorhombic phase (phase B), which is activeand selective; and a third phase (phase C) which is still poorlycharcterized. It is desirable to form the orthorhombic phase (phase B)selectively.

Two methods of forming the orthorhombic phase (phase B) withsubstantially reduced content of the hexagonal phase (phase A) areknown. The first method involves the extraction of a mixed phasecatalyst with a suitable solvent. In particular, Japanese Laid-OpenPatent Application Publication No. 10-330343 discloses the washing of amixed metal oxide of the formulaMo_(a)V_(b)Sb_(c)X_(x)O_(n)

-   -   wherein X is at least one metal element selected from Ti, Zr,        Nb, Ta, Cr, W, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Zn,        In, Sn, Pb, Bi, Ce and alkaline earth metals,    -   wherein, when a=1, 0.02≦b≦0.99, 0.001≦c≦0.9, 0≦x≦0.89,        0.1≦c/b≦0.80 and n is a value determined by the oxidation state        of the other elements,        with a solvent selected from aqueous oxalic acid, ethylene        glycol or aqueous hydrogen peroxide. The so-formed catalyst is        used for the ammoxidation of alkanes to form nitriles. Japanese        Laid-Open Patent Application Publication No. 11-043314 discloses        the washing of a mixed metal oxide of the formula        Mo_(a)V_(b)Sb_(c)X_(x)O_(n)    -   wherein X is at least one metal element selected from Ti, Zr,        Nb, Ta, Cr, W, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Zn,        In, Sn, Pb, Bi, Ce and alkaline earth metals,    -   wherein, when a=1, 0.02≦b≦0.99, 0.001≦c≦0.9,0≦x≦0.89,        0.1≦c/b≦0.80 and n is a value determined by the oxidation state        of the other elements,        with at least one solvent selected from an aqueous solution of        an organic acid, an alcohol, an aqueous solution of an inorganic        acid or an aqueous solution of hydrogen peroxide. The so-formed        material is indicated to be useful in such applications as        electronic materials, electrode materials, mechanical inorganic        materials and as catalysts in petrochemistry, etc. In        particular, use as a catalyst in the oxidative dehydrogenation        of ethane to produce ethylene is exemplified. While this        methodology allows isolation of the orthorhombic phase, it is        undesirable because about one-third of the original sample is        lost in the extraction. The second method involves the        hydrothermal synthesis of the catalyst precursor (Watanabe, et        al., “New Synthesis Route for Mo—V—Nb—Te Mixed Oxides Catalyst        for Propane Ammoxidation”, Applied Catalysis A: General, Vol.        194-195, pgs. 479-485 (2000)). This gives, after calcination, a        product enriched in the orthorhombic phase (phase B) but still        containing the hexagonal phase (phase A).

It has now been found that the orthorhombic phase (phase B) can beprepared selectively, in quantitative yield, by seeding the catalystprecursor solution with orthorhombic phase (phase B) material.

SUMMARY OF THE INVENTION

Thus, in a first aspect, the present invention provides a novel catalystcomponent which consists of the orthorhombic phase of a mixed metaloxide of the formulaA_(a)V_(b)N_(c)X_(d)O_(e)

-   -   wherein A is at least one element selected from the group        consisting of Mo and W, N is at least one element selected from        the group consisting of Te and Se, and X is at least one element        selected from the group consisting of Nb, Ta, Ti, Al, Zr, Cr,        Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B, In, As, Ge, Sn, Li, Na, K,        Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, P, Pm, Eu, GA, Dy,        Ho, Er, Tm, Yb, Lu, Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd, Y, Sm,        Tb, Br, Cu and Sc,    -   wherein, when a=1, b=0.01 to 1, c=0.01 to 1, d=0.01 to 1 and e        is dependent on the oxidation state of the other elements.

In a second aspect, the present invention provides a process forpreparing an orthorhombic phase mixed metal oxide catalyst, said processcomprising:

(a) admixing compounds of elements A, V, N and X and at least onesolvent to form a first mixture,

-   -   wherein A is at least one element selected from the group        consisting of Mo and W, N is at least one element selected from        the group consisting of Te, Se and Sb, and X is at least one        element selected from the group consisting of Nb, Ta, Ti, Al,        Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B, In, Ce, As, Ge, Sn,        Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Ag, Pb, P,        Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Au, Ag, Re, Pr, Zn, Ga, Pd,        Ir, Nd, Y, Sm, Tb, Br, Cu and Sc,    -   wherein A, V, N and X are present in such amounts that the        atomic ratio of A:V:N:X is a:b:c:d, and    -   wherein, when a=1, b=0.01 to 1, c=0.01 to 1 and d=0.01 to 1;

(b) admixing a seeding effective amount of an orthorhombic phase mixedmetal oxide, substantially free of hexagonal phase mixed metal oxide,with said first mixture to form a second mixture,

(c) removing said at least one solvent from said second mixture to forma catalyst precursor; and

(d) calcining said catalyst precursor to obtain said orthorhombic phasemixed metal oxide catalyst substantially free of hexagonal phase.

In a third aspect, the present invention provides a process forproducing an unsaturated carboxylic acid, which comprises subjecting analkane or a mixture of an alkane and an alkene to a vapor phasecatalytic oxidation reaction in the presence of a catalyst produced by aprocess in accord with the present invention.

In a fourth aspect, the present invention provides a process forproducing an unsaturated nitrile, which comprises subjecting an alkane,or a mixture of an alkane and an alkene, and ammonia to a vapor phasecatalytic oxidation reaction in the presence of a catalyst produced by aprocess in accord with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The mixed metal oxides prepared by the process of the present inventionhave the empirical formulaA_(a)V_(b)N_(c)X_(d)O_(e)wherein A is at least one element selected from the group consisting ofMo and W, N is at least one element selected from the group consistingof Te, Sb and Se, and X is at least one element selected from the groupconsisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B,In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb,P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Au, Ag, Re, Pr, Zn, Ga, Pd, Ir,Nd, Y, Sm, Tb, Br, Cu and Sc; andwherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 0.1 and e isdependent on the oxidation state of the other elements.

Preferably, when a=1, b=0.1 to 0.5, c=0.05 to 0.5 and d=0.01 to 0.5.More preferably, when a=1, b=0.15 to 0.45, c=0.05 to 0.45 and d=0.01 to0.1. The value of e, i.e. the amount of oxygen present, is dependent onthe oxidation state of the other elements in the catalyst. However, e istypically in the range of from 3 to 4.7.

The novel mixed metal oxides of the present invention have the empiricalformulaA_(a)V_(b)N_(c)X_(d)O_(e)wherein A is at least one element selected from the group consisting ofMo and W, N is at least one element selected from the group consistingof Te and Se, and X is at least one element selected from the groupconsisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B,In, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb,P. Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Au, Ag, Re, Pr, Zn, Ga, Pd, Ir,Nd, Y, Sm, Tb, Br, Cu and Sc; andwherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 0.1 and e isdependent on the oxidation state of the other elements.

Preferred mixed metal oxides have the empirical formulaeMo_(a)V_(b)Te_(c)Nb_(d)O_(e) or W_(a)V_(b)Te_(c)Nb_(d)O_(e) wherein a,b, c, d and e are as previously defined.

The orthorhombic phase mixed metal oxide substantially free of hexagonalphase may be prepared in the following manner.

In a first step, a solution may be formed by admixing compoundscontaining elements A, V, N and X, as previously defined, preferably atleast one of which contains oxygen, and at least one solvent inappropriate amounts to form the solution.

Suitable solvents include water; alcohols including, but not limited to,methanol, ethanol, propanol, and diols, etc.; as well as other polarsolvents known in the art. Generally, water is preferred. The water isany water suitable for use in chemical syntheses including, withoutlimitation, distilled water and de-ionized water. The amount of waterpresent is preferably an amount sufficient to keep the elementssubstantially in solution long enough to avoid or minimize compositionaland/or phase segregation during the preparation steps. Accordingly, theamount of water will vary according to the amounts and solubilities ofthe materials combined. However, as stated above, the amount of water ispreferably sufficient to ensure an aqueous solution is formed, at thetime of mixing.

For example, when a mixed metal oxide of the formulaMo_(a)V_(b)Te_(c)Nb_(d)O_(e) wherein the element A is Mo, the element Nis Te and the element X is Nb, is to be prepared, an aqueous solution ofniobium oxalate may be added to an aqueous solution of ammoniumheptamolybdate, ammonium metavanadate and telluric acid, so that theatomic ratio of the respective metal elements would be in the prescribedproportions.

Once the aqueous solution has been formed, it may be seeded by theaddition of a seeding effective amount of an orthorhombic phase mixedmetal oxide seed which is substantially free of hexagonal phase mixedmetal oxide. (By a seeding effective amount is meant an amount of seedmaterial effective to cause nucleation of the orthorhombic phase, e.g.,0.01% by weight of seed material based on the total weight of thesolution being seeded. By an orthorhombic phase mixed metal oxide whichis substantially free of hexagonal phase mixed metal oxide is meant amaterial that contains not less than 90% by weight of orthorhombic phasebased on the total weight of the material.) Such an orthorhombic phasemixed metal oxide seed substantially free of hexagonal phase mixed metaloxide may be obtained by any method known to the art.

Preferably, the orthorhombic phase mixed metal oxide substantially freeof hexagonal phase mixed metal oxide to be used as seed may be preparedby:

taking a mixed metal oxide having the empirical formulaA_(a)V_(b)N_(c)X_(d)O_(e)

-   -   wherein A is at least one element selected from the group        consisting of Mo and W, N is at least one element selected foin        the group consisting of Te, Se and Sb, and X is at least one        element selected from the group consisting of Nb, Ta, Ti, Al Zr,        Cr, Mn, Fe, Ru, Co, Rib, Ni, Pt, Bi, B, In, Ce, As, Ge, Sn, Li,        Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, P. Pm, Eu,        Gd, Dy, Ho, Er, Tm, Yb, Lu, Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd,        Y, Sm, Th, Br, Cu and Sc, and    -   wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, d=0.01 to 1.0        and e is dependent on the oxidation state of the other elements;

contacting the mixed metal oxide with a liquid contact member selectedfrom the group consisting of organic acids, alcohols, inorganic acidsand hydrogen peroxide to form a contact mixture; and

recovering insoluble material, from the contact mixture, to obtain theorthorhombic phase mixed metal oxide seed substantially free ofhexagonal phase mixed metal oxide.

Alternatively, the orthorhombic phase mixed metal oxide substantiallyfree of hexagonal phase mixed metal oxide, to be used as seed, may beprepared by:

admixing compounds of elements A, V, N, X and at least one solvent toform a first mixture,

-   -   wherein A is at least one element selected from the group        consisting of Mo and W, N is at least one element selected from        the group consisting of Te, Se and Sb, and X is at least one        element selected from the group consisting of Nb, Ta, Ti, Al Zr,        Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B, In, Ce, As, Ge, Sn, Li,        Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr. Ba, Ra, Hf, Pb, P, Pm, Eu,        Gd, Dy, Ho, Er, Tm, Yb, Lu, Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd,        Y, Sm, Tb, Br, Cu and Sc,    -   wherein A, V, M and X are present in such amounts that the        atomic ratio of A:V:N:X is a:b:c:d, and    -   wherein, when a=1, b=0.01 to 1.0, c=0.01 to 1.0 and d=0.01 to        1.0;

removing the at least one solvent from the first mixture to form a firstprecursor;

calcining the first precursor to form a first calcined precursor;

contacting the first calcined precursor with a liquid contact memberselected from the group consisting of organic acids, alcohols, inorganicacids and hydrogen peroxide to form a contact mixture; and

recovering insoluble material, from the contact mixture, to obtain theorthorhombic phase mixed metal oxide seed substantially free ofhexagonal phase mixed metal oxide.

The contacting of the mixed metal oxide or the first calcined precursorwith a liquid contact member selected from the group consisting oforganic acids, alcohols, inorganic acids and hydrogen peroxide may beeffected without any particular restrictions so long as the hexagonalphase (phase A) is substantially removed from the mixed metal oxide orthe first calcined precursor. In this regard, the liquid contact memberis normally used in an amount of 1 to 100 times the volume of the mixedmetal oxide or the first calcined precursor, preferably 3 to 50 timesthe volume, more preferably 5 to 25 times the volume. Contacting atelevated temperatures will remove the hexagonal phase (phase B) morerapidly. However, if prolonged contact time is not a consideration,contacting at room temperature or below may be utilized. Normally,contact temperatures of room temperature to 100° C. are utilized,preferably 50° C. to 90° C., more preferably 60° C. to 80° C. Aspreviously noted, contact time will be affected by the temperature atwhich the contacting is carried out. Normally, contact times of 1 to 100hours are utilized, preferably 2 to 20 hours, more preferably 5 to 10hours. The contact mixture is preferably agitated during the contacting.

There are no particular restrictions upon the organic acids which may beused as the liquid contacting member. For example, oxalic acid, formicacid, acetic acid, citric acid and tartaric acid may be used, however,oxalic acid is preferred. If the organic acid is a liquid, it may beused as is or in an aqueous solution. If the organic acid is a solid, itis used in an aqueous solution. When using aqueous solutions, there areno particular restrictions on the concentration of the organic acid.Normally, the concentration of the organic acid in the aqueous solutioncan vary from 0.1 to 50% by weight, preferably 1 to 15% by weight.

There are no particular restrictions upon the alcohols which may be usedas the liquid contacting member. For example, methanol, ethanol,propanol, butanol, hexanol and diols may be utilized, however, alcoholshaving one to four carbon atoms are preferred, with ethylene glycolbeing particularly preferred. The alcohols may be utilized in the formof aqueous solutions, but, if so, the water content should be held to20% by weight or less for the best effectiveness.

Similarly, there are no particular restrictions upon the inorganic acidswhich may be used as the liquid contacting member. For example, nitricacid, sulfuric acid, phosphoric acid, hydrochloric acid, perchloricacid, chloric acid and hypochlorous acid may be used, however, the useof nitric acid is especially preferrred. The inorganic acids aretypically used as aqueous solutions with concentrations of the acids inthe range of from 0.1 to 50% by weight, preferably from 0.1 to 10% byweight.

When hydrogen peroxide is utilized as the liquid contacting member, itis used in the form of an aqueous solution having a concentration in therange of from 0.1 to 50% by weight, preferably from 1 to 10% by weight.

After contacting with the liquid contacting member, insoluble materialis recovered from the so-formed contact mixture for use as seedmaterial. The insoluble material may be recovered by any conventionalliquid-solid separation method, e.g., centrifugation or filtration. Ifthe contacting was conducted at elevated temperature, the contactmixture may be cooled prior to recovery of the insoluble material.

After the solution has been seeded, the solvent is removed by anysuitable method, known in the art, to form a catalyst precursor. Suchmethods include, without limitation, vacuum drying, freeze drying, spraydrying, rotary evaporation and air drying.

For example, in the case of water being the solvent: Vacuum drying isgenerally performed at pressures ranging from 10 mmHg to 500 mmHg.Freeze drying typically entails freezing the solution, using , forinstance, liquid nitrogen, and drying the frozen solution under vacuum.Spray drying is generally performed under an inert atmosphere such asnitrogen or argon, with an inlet temperature ranging from 125° C. to200° C. and an outlet temperature ranging from 75° C. to 150° C. Rotaryevaporation is generally performed at a bath temperature of from 25° C.to 90° C. and at a pressure of from 10 mmHg to 760 mmHg, preferably at abath temperature of from 40° C. to 90° C. and at a pressure of from 10mmHg to 350 mmHg, more preferably at a bath temperature of from 40° C.to 60° C. and at a pressure of from 10 mmHg to 40 mmHg. Air drying maybe effected at temperatures ranging from 25° C. to 90° C. Rotaryevaporation or air drying are generally preferred.

Once obtained, the catalyst precursor is calcined. (The calcinationconditions disclosed hereinafter may also be used in the formation ofthe seed material as set forth above.) The calcination may be conductedin an oxygen-containing atmosphere or in the substantial absence ofoxygen, e.g., in an inert atmosphere or in vacuo. The inert atmospheremay be any material which is substantially inert, i.e., does not reactor interact with, the catalyst precursor. Suitable examples include,without limitation, nitrogen, argon, xenon, helium or mixtures thereof.Preferably, the inert atmosphere is argon or nitrogen. The inertatmosphere may flow over the surface of the catalyst precursor or maynot flow thereover (a static environment). When the inert atmospheredoes flow over the surface of the catalyst precursor, the flow rate canvary over a wide range, e.g., at a space velocity of from 1 to 500 hr⁻¹.

The calcination is usually performed at a temperature of from 350° C. to850° C., preferably from 400° C. to 700° C., more preferably from 500°C. to 650° C. The calcination is performed for an amount of timesuitable to form the aforementioned catalyst. Typically, the calcinationis performed for from 0.5 to 30 hours, preferably from 1 to 25 hours,more preferably for from 1 to 15 hours, to obtain the desired promotedmixed metal oxide.

In one mode of operation, the catalyst precursor is calcined in twostages. In the first stage, the catalyst precursor is calcined in anoxidizing environment (e.g. air) at a temperature of from 200° C. to400° C., preferably from 275° C. to 325° C. for from 15 minutes to 8hours, preferably for from 1 to 3 hours. In the second stage, thematerial from the first stage is calcined in a non-oxidizing environment(e.g., an inert atmosphere) at a temperature of from 500° C. to 750° C.,preferably for from 550° C. to 650° C., for 15 minutes to 8 hours,preferably for from 1 to 3 hours. Optionally, a reducing gas, such as,for example, ammonia or hydrogen, may be added during the second stagecalcination.

In a preferred mode of operation, the catalyst precursor in the firststage is placed in the desired oxidizing atmosphere at room temperatureand then raised to the first stage calcination temperature and heldthere for the desired first stage calcination time. The atmosphere isthen replaced with the desired non-oxidizing atmosphere for the secondstage calcination, the temperature is raised to the desired second stagecalcination temperature and held there for the desired second stagecalcination time.

Although any type of heating mechanism, e.g., a furnace, may be utilizedduring the calcination, it is preferred to conduct the calcination undera flow of the designated gaseous environment. Therefore, it isadvantageous to conduct the calcination in a bed with continuous flow ofthe desired gas(es) through the bed of solid catalyst precursorparticles.

In a particularly preferred mode of operation, the catalyst precursor inthe first stage of calcination is placed in a desired flowing oxidizingatmosphere at room temperature and then raised to the first stagecalcination temperature, at a rate of from 1° C./min to 20° C./min,preferably 2°/min to 10° C./min. It is then held at the first stagecalcination temperature, in the desired flowing oxidizing atmosphere,for the desired first stage calcination time. After the desired firststage calcination time has passed, the atmosphere is replaced with adesired flowing non-oxidizing atmosphere, preferably while maintainingthe temperature at the first stage calcination temperature; thetemperature is then raised to the desired second stage calcinationtemperature at a rate of from 1° C./min to 20° C./min, preferably 2°C./min to 10° C./min. It is then held at the second stage calcinationtemperature, in the desired flowing non-oxidizing atmosphere, for thedesired second stage calcination time.

With calcination, a catalyst is formed having the formulaA_(a)V_(b)N_(c)X_(d)O_(e) wherein A, N, X, O, a, b, c, d and e are aspreviously defined.

The starting materials for the above promoted mixed metal oxide are notlimited to those described above. A wide range of materials including,for example, oxides, nitrates, halides or oxyhalides, alkoxides,acetylacetonates, and organometallic compounds may be used. For example,ammonium heptamolybdate may be utilized for the source of molybdenum inthe catalyst. However, compounds such as MoO₃, MoO₂, MoCl₅, MoOCl₄,Mo(OC₂H₅)₅, molybdenum acetylacetonate, phosphomolybdic acid andsilicomolybdic acid may also be utilized instead of ammoniumheptamolybdate. Similarly, ammonium metavanadate may be utilized for thesource of vanadium in the catalyst. However, compounds such as V₂O₅,V₂O₃, VOCl₃, VCl₄, VO(OC₂H₅)₃, vanadium acetylacetonate and vanadylacetylacetonate may also be utilized instead of ammonium metavanadate.The tellurium source may include telluric acid, TeCl₄, Te(OC₂H₅)₅,Te(OCH(CH₃)₂)₄ and TeO₂. The niobium source may include ammonium niobiumoxalate, Nb₂O₅, NbCl₅, niobic acid or Nb(OC₂H₅)₅ as well as the moreconventional niobium oxalate.

A mixed metal oxide, thus obtained, exhibits excellent catalyticactivities by itself. However, the mixed metal oxide may be converted toa catalyst having higher activities by grinding.

There is no particular restriction as to the grinding method, andconventional methods may be employed. As a dry grinding method, a methodof using a gas stream grinder may, for example, be mentioned whereincoarse particles are permitted to collide with one another in a highspeed gas stream for grinding. The grinding may be conducted not onlymechanically but also by using a mortar or the like in the case of asmall scale operation.

As a wet grinding method wherein grinding is conducted in a wet state byadding water or an organic solvent to the above mixed metal oxide, aconventional method of using a rotary cylinder-type medium mill or amedium-stirring type mill, may be mentioned. The rotary cylinder-typemedium mill is a wet mill of the type wherein a container for the objectto be ground is rotated, and it includes, for example, a ball mill and arod mill. The medium-stirring type mill is a wet mill of the typewherein the object to be ground, contained in a container is stirred bya stirring apparatus, and it includes, for example, a rotary screw typemill, and a rotary disc type mill.

The conditions for grinding may suitably be set to meet the nature ofthe above-mentioned promoted mixed metal oxide, the viscosity, theconcentration, etc. of the solvent used in the case of wet grinding, orthe optimum conditions of the grinding apparatus. Improvement in thecatalytic performance may occur due to such grinding.

Further, in some cases, it is possible to further improve the catalyticactivities by further adding a solvent to the ground catalyst precursorto form a solution or slurry, followed by drying again. There is noparticular restriction as to the concentration of the solution orslurry, and it is usual to adjust the solution or slurry so that thetotal amount of the starting material compounds for the ground catalystprecursor is from 10 to 60 wt %. Then, this solution or slurry is driedby a method such as spray drying, freeze drying, evaporation to drynessor vacuum drying, preferably by the spray drying method. Further,similar drying may be conducted also in the case where wet grinding isconducted.

The oxide obtained by the above-mentioned method may be used as a finalcatalyst, but it may further be subjected to heat treatment usually at atemperature of from 200° to 700° C. for from 0.1 to 10 hours.

The mixed metal oxide thus obtained may be used by itself as a solidcatalyst, but may be formed into a catalyst together with a suitablecarrier such as silica, alumina, titania, aluminosilicate, diatomaceousearth or zirconia. Further, it may be molded into a suitable shape andparticle size depending upon the scale or system of the reactor.

Alternatively, the metal components of the presently contemplatedcatalyst may be supported on materials such as alumina, silica,silica-alumina, zirconia, titania, etc. by conventional incipientwetness techniques. In one typical method, solutions containing themetals, after being seeded with the orthorhombic phase mixed metal oxidematerial, are contacted with the dry support such that the support iswetted; then, the resultant wetted material is dried, for example, at atemperature from room temperature to 200° C. followed by calcination asdescribed above.

In a third aspect, the present invention provides a process forproducing an unsaturated carboxylic acid, which comprises subjecting analkane, or a mixture of an alkane and an alkene, to a vapor phasecatalytic oxidation reaction in the presence of a catalyst produced inaccord with the present invention to produce an unsaturated carboxylicacid.

In the production of such an unsaturated carboxylic acid, it ispreferred to employ a starting material gas which contains steam. Insuch a case, as a starting material gas to be supplied to the reactionsystem, a gas mixture comprising a steam-containing alkane, or asteam-containing mixture of alkane and alkene, and an oxygen-containinggas, is usually used. However, the steam-containing alkane, or thesteam-containing mixture of alkane and alkene, and the oxygen-containinggas may be alternately supplied to the reaction system. The steam to beemployed may be present in the form of steam gas in the reaction system,and the manner of its introduction is not particularly limited.

Further, as a diluting gas, an inert gas such as nitrogen, argon orhelium may be supplied. The molar ratio (alkane or mixture of alkane andalkene):(oxygen):(diluting gas):(H₂O) in the starting material gas ispreferably (1):(0.1 to 10):(0 to 20):(0.2 to 70), more preferably (1):(1to 5.0):(0 to 10):(5 to 40).

When steam is supplied together with the alkane, or the mixture ofalkane and alkene, as starting material gas, the selectivity for anunsaturated carboxylic acid is distinctly improved, and the unsaturatedcarboxylic acid can be obtained from the alkane, or mixture of alkaneand alkene, in good yield simply by contacting in one stage. However,the conventional technique utilizes a diluting gas such as nitrogen,argon or helium for the purpose of diluting the starting material. Assuch a diluting gas, to adjust the space velocity, the oxygen partialpressure and the steam partial pressure, an inert gas such as nitrogen,argon or helium may be used together with the steam.

As the starting material alkane it is preferred to employ a C₃₋₈ alkane,particularly propane, isobutane or n-butane; more preferably, propane orisobutane; most preferably, propane. According to the present invention,from such an alkane, an unsaturated carboxylic acid such as anα,β-unsaturated carboxylic acid can be obtained in good yield. Forexample, when propane or isobutane is used as the starting materialalkane, acrylic acid or methacrylic acid will be obtained, respectively,in good yield.

In the present invention, as the starting material mixture of alkane andalkene, it is preferred to employ a mixture of C₃₋₈ alkane and C₃₋₈alkene, particularly propane and propene, isobutane and isobutene orn-butane and n-butene. As the starting material mixture of alkane andalkene, propane and propene or isobutane and isobutene are morepreferred. Most preferred is a mixture of propane and propene. Accordingto the present invention, from such a mixture of an alkane and analkene, an unsaturated carboxylic acid such as an α,β-unsaturatedcarboxylic acid can be obtained in good yield. For example, when propaneand propene or isobutane and isobutene are used as the starting materialmixture of alkane and alkene, acrylic acid or methacrylic acid will beobtained, respectively, in good yield. Preferably, in the mixture ofalkane and alkene, the alkene is present in an amount of at least 0.5%by weight, more preferably at least 1.0% by weight to 95% by weight;most preferably, 3% by weight to 90% by weight.

As an alternative, an alkanol, such as isobutanol, which will dehydrateunder the reaction conditions to form its corresponding alkene, i.e.isobutene, may also be used as a feed to the present process or inconjunction with the previously mentioned feed streams.

The purity of the starting material alkane is not particularly limited,and an alkane containing a lower alkane such as methane or ethane, airor carbon dioxide, as impurities, may be used without any particularproblem. Further, the starting material alkane may be a mixture ofvarious alkanes. Similarly, the purity of the starting material mixtureof alkane and alkene is not particularly limited, and a mixture ofalkane and alkene containing a lower alkene such as ethene, a loweralkane such as methane or ethane, air or carbon dioxide, as impurities,may be used without any particular problem. Further, the startingmaterial mixture of alkane and alkene may be a mixture of variousalkanes and alkenes.

There is no limitation on the source of the alkene. It may be purchased,per se, or in admixture with an alkane and/or other impurities.Alternatively, it can be obtained as a by-product of alkane oxidation.Similarly, there is no limitation on the source of the alkane. It may bepurchased, per se, or in admixture with an alkene and/or otherimpurities. Moreover, the alkane, regardless of source, and the alkene,regardless of source, may be blended as desired.

The detailed mechanism of the oxidation reaction of the presentinvention is not clearly understood, but the oxidation reaction iscarried out by oxygen atoms present in the above promoted mixed metaloxide or by molecular oxygen present in the feed gas. To incorporatemolecular oxygen into the feed gas, such molecular oxygen may be pureoxygen gas. However, it is usually more economical to use anoxygen-containing gas such as air, since purity is not particularlyrequired.

It is also possible to use only an alkane, or a mixture of alkane andalkene, substantially in the absence of molecular oxygen for the vaporphase catalytic reaction. In such a case, it is preferred to adopt amethod wherein a part of the catalyst is appropriately withdrawn fromthe reaction zone from time to time, then sent to an oxidationregenerator, regenerated and then returned to the reaction zone forreuse. As the regeneration method of the catalyst, a method may, forexample, be mentioned which comprises contacting an oxidative gas suchas oxygen, air or nitrogen monoxide with the catalyst in the regeneratorusually at a temperature of from 300° to 600° C.

This third aspect of the present invention will be described in furtherdetail with respect to a case where propane is used as the startingmaterial alkane and air is used as the oxygen source. The reactionsystem may be a fixed bed system or a fluidized bed system. However,since the reaction is an exothermic reaction, a fluidized bed system maypreferably be employed whereby it is easy to control the reactiontemperature. The proportion of air to be supplied to the reaction systemis important for the selectivity for the resulting acrylic acid, and itis usually at most 25 moles, preferably from 0.2 to 18 moles per mole ofpropane, whereby high selectivity for acrylic acid can be obtained. Thisreaction can be conducted usually under atmospheric pressure, but may beconducted under a slightly elevated pressure or slightly reducedpressure. With respect to other alkanes such as isobutane, or tomixtures of alkanes and alkenes such as propane and propene, thecomposition of the feed gas may be selected in accordance with theconditions for propane.

Typical reaction conditions for the oxidation of propane or isobutane toacrylic acid or methacrylic acid may be utilized in the practice of thepresent invention. The process may be practiced in a single pass mode(only fresh feed is fed to the reactor) or in a recycle mode (at least aportion of the reactor effluent is returned to the reactor). Generalconditions for the process of the present invention are as follows: thereaction temperature can vary from 200° C. to 700° C., but is usually inthe range of from 200° C. to 550° C., more preferably 250° C. to 480°C., most preferably 300° C. to 400° C.; the gas space velocity, SV, inthe vapor phase reaction is usually within a range of from 100 to 10,000hr⁻¹, preferably 300 to 6,000 hr⁻¹, more preferably 300 to 2,000 hr⁻¹;the average contact time with the catalyst can be from 0.01 to 10seconds or more, but is usually in the range of from 0.1 to 10 seconds,preferably from 2 to 6 seconds; the pressure in the reaction zoneusually ranges from 0 to 75 psig, but is preferably no more than 50psig. In a single pass mode process, it is preferred that the oxygen besupplied from an oxygen-containing gas such as air. The single pass modeprocess may also be practiced with oxygen addition. In the practice ofthe recycle mode process, oxygen gas by itself is the preferred sourceso as to avoid the build up of inert gases in the reaction zone.

Of course, in the oxidation reaction of the present invention, it isimportant that the hydrocarbon and oxygen concentrations in the feedgases be maintained at the appropriate levels to minimize or avoidentering a flammable regime within the reaction zone or especially atthe outlet of the reactor zone. Generally, it is preferred that theoutlet oxygen levels be low to both minimize after-burning and,particularly, in the recycle mode of operation, to minimize the amountof oxygen in the recycled gaseous effluent stream. In addition,operation of the reaction at a low temperature (below 450° C.) isextremely attractive because after-burning becomes less of a problemwhich enables the attainment of higher selectivity to the desiredproducts. The catalyst of the present invention operates moreefficiently at the lower temperature range set forth above,significantly reducing the formation of acetic acid and carbon oxides,and increasing selectivity to acrylic acid. As a diluting gas to adjustthe space velocity and the oxygen partial pressure, an inert gas such asnitrogen, argon or helium may be employed.

When the oxidation reaction of propane, and especially the oxidationreaction of propane and propene, is conducted by the method of thepresent invention, carbon monoxide, carbon dioxide, acetic acid, etc.may be produced as by-products, in addition to acrylic acid. Further, inthe method of the present invention, an unsaturated aldehyde maysometimes be formed depending upon the reaction conditions. For example,when propane is present in the starting material mixture, acrolein maybe formed; and when isobutane is present in the starting materialmixture, methacrolein may be formed. In such a case, such an unsaturatedaldehyde can be converted to the desired unsaturated carboxylic acid bysubjecting it again to the vapor phase catalytic oxidation with thepromoted mixed metal oxide-containing catalyst of the present inventionor by subjecting it to a vapor phase catalytic oxidation reaction with aconventional oxidation reaction catalyst for an unsaturated aldehyde.

In a fourth aspect, the present invention provides a process forproducing an unsaturated nitrile, which comprises subjecting an alkane,or a mixture of an alkane and an alkene, to a vapor phase catalyticoxidation reaction with ammonia in the presence of a catalyst producedin accord with the present invention to produce an unsaturated nitrile.

In the production of such an unsaturated nitrile, as the startingmaterial alkane, it is preferred to employ a C₃₋₈ alkane such aspropane, butane, isobutane, pentane, hexane and heptane. However, inview of the industrial application of nitrites to be produced, it ispreferred to employ a lower alkane having 3 or 4 carbon atoms,particularly propane and isobutane.

Similarly, as the starting material mixture of alkane and alkene, it ispreferred to employ a mixture of C₃₋₈ alkane and C₃₋₈ alkene such aspropane and propene, butane and butene, isobutane and isobutene, pentaneand pentene, hexane and hexene, and heptane and heptene. However, inview of the industrial application of nitriles to be produced, it ismore preferred to employ a mixture of a lower alkane having 3 or 4carbon atoms and a lower alkene having 3 or 4 carbon atoms, particularlypropane and propene or isobutane and isobutene. Preferably, in themixture of alkane and alkene, the alkene is present in an amount of atleast 0.5% by weight, more preferably at least 1.0% by weight to 95% byweight, most preferably 3% by weight to 90% by weight.

The purity of the starting material alkane is not particularly limited,and an alkane containing a lower alkane such as methane or ethane, airor carbon dioxide, as impurities, may be used without any particularproblem. Further, the starting material alkane may be a mixture ofvarious alkanes. Similarly, the purity of the starting material mixtureof alkane and alkene is not particularly limited, and a mixture ofalkane and alkene containing a lower alkene such as ethene, a loweralkane such as methane or ethane, air or carbon dioxide, as impurities,may be used without any particular problem. Further, the startingmaterial mixture of alkane and alkene may be a mixture of variousalkanes and alkenes.

There is no limitation on the source of the alkene. It may be purchased,per se, or in admixture with an alkane and/or other impurities.Alternatively, it can be obtained as a by-product of alkane oxidation.Similarly, there is no limitation on the source of the alkane. It may bepurchased, per se, or in admixture with an alkene and/or otherimpurities. Moreover, the alkane, regardless of source, and the alkene,regardless of source, may be blended as desired.

The detailed mechanism of the ammoxidation reaction of this aspect ofthe present invention is not clearly understood. However, the oxidationreaction is conducted by the oxygen atoms present in the above promotedmixed metal oxide or by the molecular oxygen in the feed gas. Whenmolecular oxygen is incorporated in the feed gas, the oxygen may be pureoxygen gas. However, since high purity is not required, it is usuallyeconomical to use an oxygen-containing gas such as air.

As the feed gas, it is possible to use a gas mixture comprising analkane, or a mixture of an alkane and an alkene, ammonia and anoxygen-containing gas, However, a gas mixture comprising an alkane or amixture of an alkane and an alkene and ammonia, and an oxygen-containinggas may be supplied alternately.

When the gas phase catalytic reaction is conducted using an alkane, or amixture of an alkane and an alkene, and ammonia substantially free frommolecular oxygen, as the feed gas, it is advisable to employ a methodwherein a part of the catalyst is periodically withdrawn and sent to anoxidation regenerator for regeneration, and the regenerated catalyst isreturned to the reaction zone. As a method for regenerating thecatalyst, a method may be mentioned wherein an oxidizing gas such asoxygen, air or nitrogen monoxide is permitted to flow through thecatalyst in the regenerator usually at a temperature of from 300° C. to600° C.

This fourth aspect of the present invention will be described in furtherdetail with respect to a case where propane is used as the startingmaterial alkane and air is used as the oxygen source. The proportion ofair to be supplied for the reaction is important with respect to theselectivity for the resulting acrylonitrile. Namely, high selectivityfor acrylonitrile is obtained when air is supplied within a range of atmost 25 moles, particularly 1 to 15 moles, per mole of the propane. Theproportion of ammonia to be supplied for the reaction is preferablywithin a range of from 0.2 to 5 moles, particularly from 0.5 to 3 moles,per mole of propane. This reaction may usually be conducted underatmospheric pressure, but may be conducted under a slightly increasedpressure or a slightly reduced pressure. With respect to other alkanessuch as isobutane, or to mixtures of alkanes and alkenes such as propaneand propene, the composition of the feed gas may be selected inaccordance with the conditions for propane.

The processes of these still further aspects of the present inventionmay be conducted at a temperature of, for example, from 250° C. to 480°C. More preferably, the temperature is from 300° C. to 400° C. The gasspace velocity, SV, in the gas phase reaction is usually within therange of from 100 to 10,000 hr⁻¹, preferably from 300 to 6,000 hr⁻¹,more preferably from 300 to 2,000 hr⁻¹. As a diluent gas, for adjustingthe space velocity and the oxygen partial pressure, an inert gas such asnitrogen, argon or helium can be employed. When ammoxidation of propaneis conducted by the method of the present invention, in addition toacrylonitrile, carbon monoxide, carbon dioxide, acetonitrile,hydrocyanic acid and acrolein may form as by-products.

COMPARATIVE EXAMPLE 1

A Mo—V—Te—Nb mixed metal oxide (56.60 g) was added to a solution of 23.3g oxalic acid dihydrate in 215 g water and stirred 6 hrs at 70-80° C. Itwas cooled, filtered and dried to yield 45.28 g black solid. The X-raydiffraction pattern for this sample showed significant intensity at 2θ(2 theta) values of 28.3° and 36.2°, indicative of the hexagonal phase.

EXAMPLE 1

Ammonium heptamolybdate tetrahydrate (17.03 g), ammonium metavanadate(3.35 g) and telluric acid (5.09 g) were dissolved in 284 g water withheating. The resulting orange solution was cooled to 40° C. Oxalic aciddihydrate (0.97 g) was dissolved in a 6.5 weight percent solution ofniobium oxalate in water (99.33 g). The so-formed niobium solution wasadded to the orange solution and then 100 mg of the material as preparedin Comparative Example 1 was added to the combined solutions. Themixture was dried, first on a rotary evaporator, and then overnight,under vacuum (6 mbar). The resulting precursor was sieved to remove >50mesh fines, then calcined in a flowing atmosphere as follows: in an airatmosphere, the precursor was heated from room temperature to 275° at arate of 10° C./min and then held at 275° C. for 1 hr; the atmosphere wasswitched to argon; in an argon atmosphere, the precursor was heated from275° C. to 600° C. at a rate of 2° C./min and then held at 600° C. for 2hrs. The X-ray diffraction pattern for this sample showed very littleintensity at 2θ (2 theta) values of 28.3° and 36.2°, indicative of alack of the hexagonal phase.

1. A process for producing an unsaturated carboxylic acid which comprises subjecting an alkane, or a mixture of an alkane and an alkene, to a vapor phase catalytic oxidation reaction in the presence of an orthorhombic phase mixed metal oxide catalyst, produced by a process comprising: (a) admixing compounds of elements A, V, N and X and at least one solvent to form a solution, wherein A is at least one element selected from the group consisting of Mo and W, N is at least one element selected from the group consisting of Te, Se and Sb, and X is at least one clement selected from the group consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B, In, Ce, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, F, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Au, Ag, Re, Pr, Zn, Ga, Pd, Ir, Nd, Y, Sm, Th, Br,Cu and Sc, wherein A, V, N and X are present in such amounts that the atomic ratio of A:V:N:X is a:b:c:d, and wherein, when a=1, b=0.01 to 1, c=0.01 to 1 and d=0.01 to 1; (b) admixing a seeding effective amount of an orthorhombic phase mixed metal oxide seed, substantially free of hexagonal phase mixed metal oxide, with said solution to form a seeded solution, (c) removing said at least one solvent from said seeded solution to form a catalyst precursor; and calcining said catalyst precursor to obtain said orthorhombic phase mixed metal oxide catalyst.
 2. A process for producing an unsaturated nitrile which comprises subjecting an alkane, or a mixture of an alkane and an alkene, and ammonia to a vapor phase catalytic oxidation reaction in the presence of an orthorhombic phase mixed metal oxide catalyst, produced by a comprising: (a) admixing compounds of elements A, V, N and X and at least one solvent to form a solution, wherein A is at least one element selected from the group consisting of Mo and W, N is at least one element selected from the group consisting of Te and Se, and X is at least one element selected from the group consisting of Nb, Ta, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pt, Bi, B, In, Ce, As, Ge, Sn, Li, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Hf, Pb, P, Pm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Au, Ag, Re, Pr, Zn, Ga, Pd, In, Nd, Y, Sm, Tb, Br, Cu and Sc, wherein A, V, N and X are present in such amounts that the atomic ratio of A:V:N:X is a:b:c:d, and wherein,when a=1, b=0.001 to 1, c=0.01 to 1 and d=0.01 to 1; (b) admixing a seeding effective amount of an orthorhombic phase mixed metal oxide seed, substantially free of hexagonal phase mixed metal oxide, with said solution to form a seeded solution, (c) removing said at least one solvent from said seeded solution to form a catalyst precursor; and calcining said catalyst precursor to obtain said orthorhombic phase mixed metal oxide catalyst. 